1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus equipped with the sheet conveying apparatus.
2. Description of the Related Art
Generally, in an image forming apparatus, accuracy of image recording position with respect to a sheet (hereinafter referred to as “recording accuracy”) is one of important factors in maintaining image quality. For example, if a sheet conveyed during image formation is skewed, it is necessary to correct the skewed sheet to form an image in an appropriate position. Various sheet conveying apparatus provided with a skew correction function have been proposed for use in conventional image forming apparatus in order to improve recording accuracy (see Japanese Patent Application Laid-Open No. H09-183539).
For example, a sheet conveying apparatus described in Japanese Patent Application Laid-Open No. H09-183539 includes a plurality of conveying roller pairs installed in a sheet width direction orthogonal to a sheet conveying direction, and a rotatable shutter member is placed between the conveying roller pairs on a rotating shaft of conveying rollers. The shutter member has an abutting portion configured to be abutted by a sheet. When a leading end of the sheet abuts the abutting portion, the sheet slacks by the reaction force of the abutting portion, bending into a curve. The formation of the curve causes a leading end portion of the sheet to be adjusted parallel to the sheet width direction orthogonal to the conveying direction, thereby correcting the skew. Subsequently, when the shutter member rotates, the sheet is conveyed with the leading end of the sheet pinched in nip portions of the conveying roller pairs in a state of being parallel to the sheet width direction. That is, the sheet is conveyed with skew corrected.
In these years, further improvements in throughput are demanded of image forming apparatus and there is demand to improve sheet conveying speed and reduce gap (hereinafter referred to as “sheet gap”) between the rear end of a preceding sheet and the leading end of a subsequent sheet. Thus, after passage of the preceding sheet, the shutter members need to be returned to home position in the sheet gap which has been reduced.
Shutter members installed in a sheet conveying apparatus regarding the present invention are illustrated in FIGS. 22 and 23. As illustrated in FIGS. 22 and 23, shutter members 423 are rotatably supported on a rotating shaft 418a of conveying roller pairs 418, 419. The shutter members 423 are adapted to make a sheet pinched between the conveying pairs by rotating through a nip portion and return to a waiting position by rotating backward. Therefore, a minimum required sheet gap distance D3 is a sum of a distance D1 from a position at which a rear end of a preceding sheet S passes through abutting surfaces of the shutter members 423 to the waiting position (home position of the shutter members) P at which the sheet S undergoes a skew correction and a distance D2 over which a subsequent sheet S is conveyed to the waiting position in the meantime (see FIG. 23).
So long as the shutter members 423 reciprocate through the nip portion of the conveying roller pairs 418, 419, the distance D1 is involved, and it takes time Δt for the shutter members 423 to travel the distance D1. The distance D2 equals a distance (Δt×V) obtained by multiplying the time Δt needed for the shutter members 423 to travel the distance D1 by conveying speed V of the sheet S, meaning that the faster the conveying speed V of the sheet S, the larger the distance. Therefore, the sheet conveying apparatus has a problem in that the sheet gap increases with increases in the conveying speed of the sheet S, limiting further improvements in throughput.